As shown in Japanese patent No. 2600492, which is a counterpart of U.S. Pat. No. 5,473,887, it is well known that an exhaust purification device absorbs nitrogen oxide (NOx) when air-fuel ratio of exhaust gas is lean, and reduces the NOx when the air-fuel ratio is rich.
As a method of making the air-fuel ratio rich, a rich-combustion is known, in which a rich-gas is produced in an engine cylinder. When the rich-combustion is performed, an ignition timing, an injection pressure, a pilot injection quantity and the like are controlled differently from those in a lean-combustion in order to reduce emissions.
In order that a driver and/or passengers do not have uncomfortable feeling in between the lean-combustion and the rich-combustion, combustion noises are adjusted to be heard sensorily equal therebetween.
But, during a transition period from the lean-combustion to the rich-combustion, since the oxygen concentration (O2 concentration) in a cylinder and the other combustion condition are gradually varied, it is difficult to keep the combustion noise constant, which may make the passengers uncomfortable.